1. Field of the Invention
The present invention is related to a display apparatus where a light transmission and a light shading are controlled for every pixel to display an image, and a backlight unit and an optical sheet used for this display. An examples of such a display apparatus includes a liquid crystal display.
2. Description of the Related Art
A liquid crystal display apparatus using a transmission type liquid crystal panel has a backlight unit and a liquid crystal panel where pixels are arranged like dots. A character or an image can be displayed by controlling a light transmittance of each pixel. The following are types of backlight unit: the distribution of outgoing light is controlled by a combination of a halogen lamp, a reflection plate and a lens; and a cold-cathode tube is provided at an end surface of a light guiding body where a light emitted from a cold-cathode tube exits from a surface (a main surface) which is perpendicular to the end surface. The former is mainly used for a liquid crystal projector which needs high luminance. The latter is used for a display for a direct-view-type liquid crystal TV or a notebook computer since a display using the latter can be made thin. In a liquid crystal TV or a notebook computer, reduction of power consumption and high luminance are needed. High luminance can be realized by increasing the number of light sources such as a cold-cathode tube, however the increasing number of light sources contributes to the increase of power consumption. Therefore, it is not suitable for practical use.
An example such as this backlight unit is described as follows: as shown in FIG. 5, a light reflection plate is arranged in a back side of a light source, and an optical sheet which controls the range of the exiting light is placed. This optical sheet has a transparent substrate, a plurality of microlenses arranged on a surface in a liquid crystal panel side of this transparent substrate, light transmission parts provided at positions corresponding to the microlenses wherein the positions are in a surface of a light source side of this transparent substrate, a light reflection layer covering part of a transparent substrate surface in a light source side wherein the part is without this light transmission part, and a light scattering layer covering the light transmission part and the light reflection part. That is, at first, light emitted from a light source is diffused uniformly by the light scattering layer. An exiting light directed to the light transmission part of this diffused light is transmitted through this light transmission part. Next, the transmitted light is refracted by the microlens to be directed to a direction which is perpendicular to a surface of the optical sheet. Next, the refracted light exits from the optical sheet as a parallel light. In addition, the diffused light which is directed to a part except for the light transmission part is reflected by the light reflection plate or the light reflection layer. The reflection is repeated. Finally, the reflected light passes through the light transmission part, thereafter the light is refracted by the microlens. Then, the light exits from the optical sheet as a parallel light which is perpendicular to a surface of the optical sheet. In this way, all lights from a light source pass through the light transmission part and the microlens and exit from the optical sheet. Then, the light enters into a liquid crystal panel as a parallel light. Therefore, the utilization efficiency of light from a light source can be improved while the distribution of the light can be controlled, thereby a bright image display can be realized.
[patent document 1] JP-A-2006-106197